sixtracklib/common: Expand NS(Buffer) API
sixtracklib/common: Expand NS(Buffer) API

- Add top-level API to get the maximum number (i.e. the "capacity") of
  entities that can be stored in a Buffer

- Add top-level API to add an NS(Object) conform element into the buffer.
  Towards the end of bridging different memory regions/memory models with the
  same API at this level, a SIXTRL_ARGPTR_DEC and SIXTRL_DATAPTR_DEC pair of
  macros has been introduced -> it's not clear yet if this works but looks
  ok

- Harmonize the API with the required changes for adding NS(Particles) and
  for future beam-elements based on NS(Buffer)

SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib